3.8 Aldehydes and Ketones

Question 1

By considering the mechanism of the reaction, explain why the product ( reaction between NaBH4 and butanone) has no effect on plane polarised light.(6)

Answer 1

Stage 1: Formation of product
• Nucleophilic attack
• Planar carbonyl group
• H – attacks from either side (stated or drawn)
Stage 2: Nature of product
• Product of step 1 shown
• This exists in two chiral forms (stated or drawn)
• Equal amounts of each enantiomer / racemic mixture formed
Stage 3: Optical activity
• Optical isomers / enantiomers rotate the plane of polarised light equally in
• With a racemic / equal mixture the effects cancel

Question 2

Ethanol can be oxidised by acidified potassium dichromate(VI) to ethanoic acid in a two-step process.
ethanol ethanal ethanoic acid
(a) In order to ensure that the oxidation to ethanoic acid is complete, the reaction is carried out under reflux.
Describe what happens when a reaction mixture is refluxed and why it is necessary, in this case, for complete oxidation to ethanoic acid.

Answer 2

A mixture of liquids is heated to boiling point for a prolonged time
1
Vapour is formed which escapes from the liquid mixture, is changed back into liquid and returned to the liquid mixture
1
Any ethanal and ethanol that initially evaporates can then be oxidised

Question 3

Write a half-equation for the overall oxidation of ethanol into ethanoic acid.

Answer 3

CH3CH2OH + H2O → CH3COOH + 4H+ + 4e–

Question 4

The boiling points of the organic compounds in a reaction mixture are shown in the following table.

Compound	ethanol	( 78 Celsius) 
ethanal	(21 Celsius )
ethanoic acid ( 118)

Use these data to describe how you would obtain a sample of ethanal from a mixture of these three compounds. Include in your answer a description of the apparatus you would use and how you would minimise the loss of ethanal. Your description of the apparatus can be either a description in words or a labelled sketch.

Answer 4

Mixture heated in a suitable flask / container
A labelled sketch illustrating these points scores the marks
1
With still head containing a thermometer
1
Water cooled condenser connected to the still head and suitable cooled collecting vessel
1
Collect sample at the boiling point of ethanal
1
Cooled collection vessel necessary to reduce evaporation of ethanal

Question 5

Colour change for acidified potassium dichromate?

Answer 5

Orange to green